The present invention relates in general to pulse generators utilizing diode switches to switch load currents through a load resistor to produce a stepped output voltage.
The bandwidth of an amplifier (or any other electronic device) can be determined by measuring the output of the amplifier in response to a range of input signal frequencies but such a method can be time consuming. Fortunately the bandwidth of an amplifier can be more quickly determined by observing its response to a square wave signal since the rise time of the amplifier output voltage in response to an abrupt transition in an input voltage is inversely proportional to the amplifier bandwidth. The accuracy of the measurement depends on the abruptness of the input voltage transition provided by the square wave signal and also on the flatness of the square wave signal before and after the transition.
One method of producing a relatively fast voltage transition utilizes a Schottky diode switch to divert a constant current away from a load resistor. As illustrated in FIG. 1, the diode switch comprises a pair of Schottky diodes Da and Db having cathodes connected to a current source Is. The anode of diode Db is connected to ground through a load resistor R.sub.o while the anode of diode Da is connected to a negative voltage -V through another resistor R and to a source of positive voltage +V through a high speed switch S. When switch S is open, the -V source forward biases diode Db and reverse biases diode Da such that current Is is drawn through resistor R.sub.o to produce a negative output voltage Vo. When switch S is closed, voltage source +V forward biases diode Da and reverse biases diode Db so that current Is is supplied from +V through diode switch S and diode Da rather than from ground through resistor R.sub.o and diode Db. As the rise time of the leading edge of the pulse applied to diode Da produced by closing switch S is decreased, the transition of Vo from a positive voltage to a zero voltage becomes more abrupt. However diodes Da and Db include inherent capacitances which differentiate the square wave pulse from switch S1 to produce a small current which is fed through diodes Da and Db to the load resistor R.sub.o and this current causes the output voltage Vo to ring following a transition. As the rise time of the leading edge of the pulse applied to Da is increased, the amount of ringing in Vo after Vo switches state is also increased. Thus the switching speed of this circuit is limited by the amount of ringing that can be tolerated in Vo following a state transition.
What is needed is a pulse generator for producing an abrupt state transition in an output voltage wherein the output voltage is flat following the state transition.